The aviation industry is developing new approach and landing systems based on the global navigation satellite system (GNSS). A GNSS Landing System (GLS) integrates satellite and ground-based navigation information to provide position information required for approach and landing guidance. The potential benefits of the GLS include significantly improved take-off and landing capability at airports worldwide, reduced cost, and improved instrument approach service at additional airports and runways. GLS may eventually replace the Instrument Landing System (ILS) which is the current standard for civil precision approach and landing systems.
One of most demanding aviation applications for GNSS is low visibility approach and landing. The most demanding of these precision approach and landing operations in the US are called “Category I” (CAT I), “Category II” (CAT II) and “Category III” (CAT III). During a CAT I operation, a pilot may continue a descent to land, with a runway visual range (RVR) as low as 1800 ft, until the aircraft is only 200 ft above the runway surface. At this time if he decides there are inadequate visual cues to safely land the aircraft, he aborts the landing attempt. During a CAT II operation, a pilot may continue, with a RVR as low as 1200 ft, until only 100 ft above the surface. During a demanding CAT III operation, a pilot may land with an RVR of only 150 ft. Highly reliable and accurate positioning and navigation are required throughout these operations.
GLS is based on a form of GNSS augmentation known as the Ground Based Augmentation System (GBAS). The International Civil Aviation Organization (ICAO) currently defines Standards and Recommended Practices (SARPs) for a GBAS architecture that is suitable for airplane approach and landing operations in Category I (CAT I) conditions. However, the currently defined GBAS architecture does not support approach and landing operations in Category II/III (CAT II/III) conditions.
Definition of a GBAS architecture to support approach and landing operations in Category II/III conditions has been difficult. This is because the current GPS satellite constellation alone does not provide with high availability a sufficiently robust geometry between the satellites and user needed to provide the sufficient accuracy and integrity performance to support this type of operation.
A method that has been suggested for augmenting the current GPS constellation is to include pseudo-satellites (pseudolites). A pseudolite is a transmitter placed on the ground at a known location. The pseudolite broadcasts a signal similar to the GPS navigation satellite signal and can provide a highly accurate range measurement. However, pseudolites can interfere with the normal reception of GPS signals. Also, pseudolites are potentially expensive and, at a minimum, entail increased ground infrastructure costs.
Another potential solution for augmenting the current GPS constellation relies on tight integration of GPS receivers and on-board inertial systems (INS). GPS/INS integration is also relatively expensive. Also, this solution is still ultimately limited by the coverage offered by the GPS satellite constellation.
Therefore, there exists an unmet need for a satellite-supported landing system that meets requirements to perform CAT II/III type landing approaches.